The present invention relates to a voltage dividing integrated circuit device and, more particularly, a voltage dividing integrated circuit device suitable for providing equally divided potentials necessary for dynamically driving a liquid crystal display device.
An AC driving system is desirable for driving a liquid crystal display device due to its chemical characteristic. That is, in order to prolong the life of liquid crystal display device, it is necessary to cancel out the influence of a positive applied voltage on the liquid crystal display device by application of a negative voltage and vice versa. The dynamic driving system in which segment electrodes of the display device are scanned on the basis of a time division, generally uses voltage levels of three or more because the liquid crystal device has a relatively long response time. In the drive voltages, the maximum and the minimum voltage levels may be positive and negative potential levels of a power supply source. The intermediate potential levels may be provided by division of the power source voltage. In electronic digital apparatus, it is desirable that the voltage dividing circuit be integrated on a single semiconductor chip, together with a logic circuit section and a liquid crystal drive circuit.
Referring to FIG. 1, there is shown an example of an integrable voltage dividing circuit which is disclosed in the copending application Ser. No. 818,295, filed Sept. 22, 1977, entitled "DISPLAY DEVICE DRIVING VOLTAGE PROVIDING CIRCUIT" and assigned to the same assignee as the present application.
In this example which is directed to a 1/3 duty and 1/3 prebias display system, a voltage dividing circuit 11 connected between power supply terminals 12 and 13 connected across a -E volt power source 14 provides to a driver circuit 15 the minimum potential -E.sub.O, the maximum potential 0 volt, and equally divided voltages -3/4E.sub.O, -1/2E.sub.O, and -1/4E.sub.O. The driver circuit 15 is connected to receive control signals h.sub.1, h.sub.2 and h.sub.3 and data signals from a CMOS logic circuitry 17 to provide to an 8-digit liquid crystal display device 16 scanning pulses H.sub.1, H.sub.2 and H.sub.3, segment signals .alpha..sub.1 to .alpha..sub.8, .beta..sub.1 to .beta..sub.8 and .gamma..sub.1 to .gamma..sub.8.
The voltage dividing circuit 11 is comprised of four voltage dividing units U.sub.1 to U.sub.4 connected in series between the power supply terminals 12 and 13. The voltage dividing unit U.sub.1 includes resistive elements R.sub.11 and R.sub.21 and a P channel MOS transistor P.sub.1 ; the dividing unit U.sub.2 resistive elements R.sub.12 and R.sub.22 and a P channel MOS transistor P.sub.2 ; the dividing unit U.sub.3 resistive elements R.sub.13 and R.sub.23 and an N channel MOS transistor N.sub.1 ; the dividing unit U.sub.4 resistive elements R.sub.14 and R.sub.24 and an N channel MOS transistor N.sub.2. A clock pulse .phi. is applied to the gates of the P channel MOS transistors P.sub.1 and P.sub.2. A clock pulse .phi. is applied to the gates of the N channel MOS transistors N.sub.1 and N.sub.2. For obtaining the equally divided potentials, the resistors R.sub.11 to R.sub.14 have equal resistance values and similarly resistors R.sub.21 to R.sub.24 have equal resistance values. The resistance value of the resistors R.sub.11 to R.sub.14 may be ranged from 100 to 400k.OMEGA. and the resistance value of the resistors R.sub.21 to R.sub.24 may be approximately 10k.OMEGA..
The voltage dividing circuit 11 is formed on a single semiconductor chip, together with the CMOS logic circuit 17 and the driver circuit 15. In the integrated circuit, a resistor is made of a semiconductor region of a conductivity type opposite to that of a semiconductor region in which the resistor is formed. In order that a plurality of resistive regions have the same resistance values, these regions are generally formed to have the same dimensions. For a high density integrated circuit it is desirable to make the occupied areas of the high resistance elements R.sub.11 to R.sub.14 as small as possible. This necessitates, however, increase of sheet resistance of the resistive region. For this reason, the resistive region is formed by a semiconductor region having a low concentration of impurities. In this case, the voltage-current characteristic of the resistor region with a low concentration of impurities depends largely on a potential difference between the resistor region and the substrate, compared to a resistive region with a high concentration of impurities. In general, the resistive region with a low concentration of impurities has a saturation characteristic in which current flowing through the resistor region does not change linearly with change of an applied voltage but saturates. The degree of the current saturation becomes larger as the potential difference between the resistive region and the substrate is larger. Therefore, with respect to the resistors R.sub.11 to R.sub.14 shown in FIG. 1, the potential difference becomes larger in the order of R.sub.11, R.sub.12, R.sub.13 and R.sub.14. As a result, where the resistive regions R.sub.11 to R.sub.14 have the same dimensions, even if the same voltage is applied across each resistive region, the resistive regions R.sub.11 to R.sub.14 would have different resistance values. This implies that it fails to obtain desired equally divided potentials for driving liquid crystal and the life of the liquid crystal is shortened.